Optical fiber drawing method and drawing furnace

ABSTRACT

A fiber drawing method according to the present invention is a drawing method of optical fiber for drawing an optical fiber  14  from one end of a fiber preform  13  by softening with heat, wherein the fiber preform  13  is set in a semi-closed space  10, 20  opening in part at a lower end in a fiber drawing furnace, the fiber preform  13  is heated by a heater  15  disposed on the lower end side of this semi-closed space  10, 20,  and fiber drawing is carried out with adjusting a quantity of heat dissipation from the upper portion  20  of this semi-closed space.

TECHNICAL FIELD

The present invention relates to a fiber drawing method of optical fibercapable of suppressing diameter fluctuations and to optical fiberdrawing furnaces used of in this method.

BACKGROUND ART

Optical fibers are normally fabricated by softening with heat anddrawing from an optical fiber preform shaped like a rod in the opticalfiber drawing furnace. In order to reduce the production cost of opticalfibers, it is effective to increase the length of the preform andthereby decrease the number of replacement works thereof. At the presenttime it is possible to make an optical fiber of the total length severalhundred kilometers by single optical fiber drawing.

The drawing furnaces have also been improved in order to implementstable drawing of such an elongated fiber preform. The drawing furnacedisclosed in Japanese Patent Application Laid-Open No. H09-2832 (whichwill be called hereinafter a prior art) is an example of such drawingfurnaces for drawing of the elongated fiber preform. This drawingfurnace is constructed in such structure that a preform containercylinder is coupled to an upper portion of a furnace core tube providedwith a heater surrounding it. Then the fiber preform is set in thepreform container and the lower end thereof is guided into the furnacecore tube. On the other hand, an inert gas such as helium or nitrogen issupplied from the upper end of the preform container. This keeps thefurnace core tube and a semi-closed space above it (which will bereferred to hereinafter simply as a semi-closed space) in anon-oxidizing atmosphere and the fiber preform is heated to soften fromthe lower end by the heater, followed by drawing.

During the drawing operation of optical fiber, the fiber preform becomesshorter and shorter with progress of fiber drawing. In the case of thedrawing furnace with the preform container coupled, which is disclosedin this prior art, as the fiber preform becomes smaller with progress offiber drawing, the space gradually becomes wider between the preformcontainer and the fiber preform. It makes the inert gas in this spaceeasier to flow and also increases a temperature difference between theinert gas in this space and the inert gas present between the furnacecore tube and the fiber preform under drawing, so as to cause convectionof the inert gas in the semi-closed space.

Occurrence of such convection leads to instable flow of the atmospherenear the lower end of the fiber preform in the softened state with heat,i.e., near the outside of the opening end of the semi-closed space. Itcan affect the optical fiber under drawing so as to make the diameterfluctuations of optical fiber considerably large, thus making itdifficult to obtain products with desired quality.

As countermeasures against it, the prior art discloses the technology ofdisposing an annular auxiliary heater around the upper end of thepreform container and heating and retaining the inside of the upper endof the preform container at several hundred degrees. It is describedthat this technology can prevent occurrence of the convection in thesemi-closed space and thus permit the optical fiber to be drawn insteady diameter.

DISCLOSURE OF THE INVENTION

With the drawing furnace as above described, further increase in thelength of the fiber preform will result in also extending the preformcontainer housing it. It also increases the volume of the semi-closedspace. It is obvious that the heating region by the auxiliary heateralso has to be elongated in order to prevent the unwanted convection inthe semi-closed space.

The fiber preform of this kind is supported so as to be suspended from asupport rod having the diameter smaller than the outside diameter of thepreform in the preform container. The preform has a shoulder graduallydecreasing its diameter toward the end, near a joint with the supportrod. When the fiber preform is heated for drawing, this shoulderradiates a large quantity of heat, which also heats up the preformcontainer facing it. With the elongation of the fiber preformaccompanied by the expansion of the heating region inside the preformcontainer, there is a possibility of overheating the internal wall ofthe preform container and eventually melting it. The shoulder of thefiber preform can also soften by the overheat, whereby the fiber preformundergoes axial extension in the shoulder part because of the weight ofthe fiber preform itself, so as to raise a possibility of failure innormal drawing of optical fiber.

In view of the above problem, an object of the present invention is toprovide an optical fiber drawing method and an optical fiber drawingfurnace capable of surely producing the optical fiber in steady diametereven in cases using the elongated fiber preform.

In order to accomplish the above object, an optical fiber drawing methodaccording to the present invention is a drawing method of optical fibercomprising steps of setting an optical fiber preform in a furnace coretube and a preform container connected to an upper portion of thefurnace core tube and drawing an optical fiber from one end of thepreform by softening with heat, wherein an upper portion of the preformcontainer is provided with an auxiliary heater and cooling means forcooling the upper portion of the preform container, and the drawing stepincludes adjusting a cooling quantity by said cooling means.

Namely, a drawing furnace used in this drawing method is a fiber drawingfurnace comprising a furnace core tube through which an optical fiberpreform penetrates vertically, a heater disposed around this furnacecore tube and a preform container connected to an upper portion of thefurnace core tube so as to be integral with the furnace core tube toform a semi-closed space opening in part at a lower end, for housing thefiber preform inside, the fiber drawing furnace further comprises anauxiliary heater disposed at an upper portion of the preform containerand cooling means for cooling the upper portion of the preformcontainer.

The present invention permits the temperature difference to be reducedin the space of clearance to the fiber preform in the semi-closed spaceformed by the furnace core tube and the preform container, so as tosuppress occurrence of convection described above, even in the case ofthe elongated fiber preform. Further, cooling the upper portion of thepreform container prevents the overheat of the internal wall of thepreform container and, in turn, prevents the overheat of the shoulder ofthe fiber preform, which permits the optical fiber to be surely drawn insteady diameter and which prevents breakage of the drawing furnace.

Here the drawing furnace is preferably one further comprising at leastone temperature sensor for measuring an internal temperature of theupper portion of the preform container and adjusting the coolingquantity based on the temperature measured by the temperature sensor.

It is preferable to employ either of the following techniques for thecooling quantity from the upper portion of the preform container.

For example, the cooling quantity may be adjusted by supplying coolingair into clearance between the auxiliary heater and the outer wall ofthe preform container. Another technique is to adjust the coolingquantity by heater moving means for moving the auxiliary heater tochange the distance to the preform container. In this case, it is alsooptional to supply the cooling air into the clearance between thepreform container and the auxiliary heater, which is created by movementof the auxiliary heater.

In another technique, the auxiliary heater has a heating element and aheat insulator formed around it and difference to be reduced in thespace of clearance to the fiber preform in the semi-Closed space formedby the furnace core tube and the preform container, so as to suppressoccurrence of convection described above, even in the case of theelongated fiber preform. Further, the adjustment of the quantity of heatdissipation from the upper portion of the preform container prevents theoverheat of the internal wall of the preform container and, in turn,prevents the overheat of the shoulder of the fiber preform, whichpermits the optical fiber to be surely drawn in steady diameter andwhich prevents breakage of the drawing furnace.

Here the drawing furnace is preferably one further comprising at leastone temperature sensor for measuring an internal temperature of theupper portion of the preform container and adjusting the quantity ofheat dissipation, based on the temperature measured by the temperaturesensor.

It is preferable to employ either of the following techniques for theadjustment of the quantity of heat dissipation from the upper portion ofthe preform container.

For example, the quantity of heat radiation may be adjusted by supplyingcooling air into clearance between the auxiliary heater and the outerwall of the the cooling quantity is adjusted by moving the heatinsulator to change the distance to the preform container. In this case,it is also optional to supply the cooling air into the clearance betweenthe heat insulator and the auxiliary heater, which is created bymovement of the heat insulator.

In another technique, the furnace may further comprise a cooling fluidcirculation path which is formed around the preform container and inwhich a cooling fluid flows, and supply means for supplying the coolingfluid into the circulation path. This cooling fluid is preferably air orwater.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view to show the structure of the firstembodiment of the optical fiber drawing furnace according to the presentinvention, FIG. 2 is an enlarged view to show the major part thereof,and FIG. 3 is a cross-sectional view along a line III—III in FIG. 2.FIG. 4 is a drawing to show an example of a heater moving device.

FIG. 5 to FIG. 8 are cross-sectional views each showing the upperpreform container part and the heater moving device in other embodimentsof the optical fiber drawing furnace according to the present invention.

FIG. 9 is a cutaway view to show the structure of the major part of thefifth embodiment of the optical fiber drawing furnace according to thepresent invention, and FIG. 10 is a cross-sectional view along a lineX—X of FIG. 9.

FIG. 11 is a view to show the structure of the major part of the sixthembodiment of the optical fiber drawing furnace according to the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

The preferred embodiments of the present invention will be describedhereinafter in detail with reference to the accompanying drawings. Tofacilitate the comprehension of the explanation, the same referencenumerals denote the same parts, where possible, throughout the drawings,and a repeated explanation will be omitted.

FIG. 1 is a cross-sectional view to show the structure of the firstembodiment of the optical fiber drawing furnace according to the presentinvention, FIG. 2 an enlarged view to show the major part thereof, andFIG. 3 a cross-sectional view along a line III—III in FIG. 2

This optical fiber drawing furnace is provided with a furnace body 11made of stainless steel and lined inside with a heat insulator. Acylindrical furnace core tube 12 is disposed in the central part of thisfurnace body 11 and an annular carbon heater 15 is placed around it,i.e., between the furnace core tube 12 and the furnace body 11. Thefurnace core tube 12 and the carbon heater 15 are concentricallyarranged. The cylindrical space inside the furnace core tube 12 will becalled hereinafter a core chamber 10.

During drawing, a fiber preform 13, an upper end of which is coupledthrough a joint 17 to a lower end of support rod 16 held by a chuck notshown, is fed from the lower end side thereof along the center axis ofthe core chamber 10 and an optical fiber 14 is formed by heating anddrawing. A seal sheet 19, having a through hole 18 for letting theoptical fiber 14 pass in the center, is attached to the lower end of thefurnace body 11. When the fiber preform 13 is large, it is preferable toprovide a cylindrical furnace core tube extension instead of the sealsheet 19.

A cooling jacket connected to a refrigerant circulator not shown isincorporated in this furnace body 11 and the inside of the core chamber10 is maintained at a predetermined temperature by controlling supplyrates and temperatures of a refrigerant from the refrigerant circulatorinto the cooling jacket in combination with heating of the carbon heater15 by a control unit 29 described hereinafter.

Preform container cylinders 21, 22 made of a heat resistant alloy suchas inconel or the like are connected to the upper end of the furnacebody 11. The internal wall of the furnace core tube 12 is extended to beintegrated with the internal walls of the preform containers 21, 22,thereby forming an upper chamber 20. As a result, the furnace core tube12 and the preform containers 21, 22 form a semi-closeD space (the uppero chamber 20 and core chamber 10) having the aperture 18 at the lowerend. Here the preform container part does not always have to beconstructed in the structure of FIG. 1, but may also be constructed byconnecting three or more cylindrical members in series, or maybeintegrally formed.

A shuttering 24, which has an opening 23 of a small diameter for lettingthe support rod 16 pass so as to be slidable in the center, is attachedto the upper end of the preform container 21 to maintain airtightnessinside the upper chamber 20. Further, a gas inlet port not shown isprovided at the upper end of the preform container 21. A supply of inertgas such as helium or nitrogen is connected through a gas supply tube tothe gas inlet port. The inert gas is supplied from this inert gas supplythrough the gas supply tube and gas inlet port to the upper end of theupper chamber 20 to maintain the inside of the chambers 10 and 20 as thesemi-closed space in the inert gas atmosphere.

Around this upper preform container 21, there are auxiliary heaters 25of an arcuate shape arranged in a vertical two-stage configuration. Eachof these auxiliary heaters 25 has an iron-chromium-aluminum-basedheating wire 27 and a heat insulator 28 of ceramic fiber surroundingthis heating wire to hold it. Two halves of each heater are arranged insymmetry with the preform container 21 in between and clearance 31 isformed between them, as shown in FIG. 3. A plurality of temperaturesensors 30 for measuring the temperature of the wall of the preformcontainer 21 are placed in this part of clearance 31.

Then each of the heating wires 27 is connected to the control unit 29for controlling power supply states to them. Each of the temperaturesensors 30 described above is connected to this control unit 29 to sendeach control information thereto.

The auxiliary heaters 25 are connected to a heater moving device 33, andby actuating this heater moving device 33, the distance between theauxiliary heaters 25 and the preform container 21 can be adjustedbetween a heating position shown by chain double-dashed lines and adissipating position shown by solid lines in FIG. 3.

FIG. 4.is a view to show an example of this heater moving device. Theauxiliary heater 25 is mounted so as to be movable on rails 332 attachedto the preform container 21. Then motors 330 are driven to move theauxiliary heater 25 toward or away from the preform container 21 throughshafts 331 attached to the auxiliary heater 25. The auxiliary heater 25can be translationally moved by using linear motors as the motors 330 orby placing rack-and-pinion gears between the motors 330 and the shafts331.

Next described in detail is the operation of the present embodiment,i.e., the drawing method of optical fiber according to the presentinvention.

The upper end of the fiber preform 13 is coupled through the joint 17 tothe lower end of the support rod 16. Then this support rod 16 is held bythe chuck not shown, whereby the fiber preform 13 is set in the upperchamber 20, i.e., inside the preform containers 21, 22. Then theshuttering 24 is attached so as to allow the support rod 16 to slide inthe aperture 23.

In this state the inert gas is supplied from the inert gas supply sourcenot shown through the gas supply tube and gas inlet port into the upperchamber 20 to fill the inside of the thus-created semi-closed space withthe inert gas atmosphere.

After that, the fiber preform 13 is fed from the lower end thereof intothe core chamber 10. The fiber preform 13 is heated in the core chamber10 by the carbon heater 15 to melt and soften and then fiber drawing iscarried out to make the optical fiber 14.

In the initial stage of drawing (see FIG. 2) in which the fiber preform13 is sufficiently long and in which the shoulder 26 with diametersgradually decreasing at the upper end is located inside the upperpreform container 21, the volume of space is small above this shoulder26 in the upper chamber 20, so that great thermal convection is unlikelyto occur.

However, if the drawing should be carried out simply, the followingproblem would arise. The heat radiation from the carbon heater 15 isincident on the lower end of the fiber preform 13, passes through thecylindrical body thereof, and radiates from the shoulder 26. This causesincrease in the temperature of the internal wall of the preformcontainer 21 and could melt the preform container 21 in the worst case.Even if the melting of the preform container 21 is not encountered, thefiber preform 13 could soften in the shoulder 26 because of the increasein the temperature of the shoulder 26 and be extended by the dead weightthereof. It can result in great diameter fluctuations of the opticalfiber 14 and the fiber preform 13 itself can break at the shoulder 26 todrop in the worst case.

This phenomenon can occur when the temperature in the upper part of thepreform container 21 exceeds 800° C. In the present embodiment,therefore, the control unit 29 monitors the temperature of the preformcontainer 21 by the temperature sensors 30. When the temperaturemeasured exceeds a predetermined temperature, the control unit 29actuates the heater moving device 33 to move the auxiliary heaters 25away from the preform container 21 as shown by the solid lines in FIG.3. This creates the clearance 32 of about 4 to 5 cm between the externalwall of the preform container 21 and the auxiliary heaters 25 to promotedissipation of heat from the external wall of the preform container 21to the outside air passing this clearance 32, thereby cooling thepreform container 21 and preventing the overheat thereof.

A necessary condition is that the temperature of the preform container21 is controlled not more than 800° C., as described above, but it ispreferable to control the temperature not more than 700° C. in terms ofassurance of lifetime for the drawing furnace and stability offabrication of the optical fiber 14. In this case, preferably, thecontrol unit 29 cuts off the power supply to the auxiliary heaters 25 tobring them into a non-heating state.

The fiber preform 13 descends in the upper chamber 20 with progress ofdrawing and the shoulder 26 also descends. This results in increase inthe volume of the space above the shoulder 26 in the upper chamber 20.When the volume of the space in this portion increases considerably, thetemperature of the gas decreases in the upper part in this space. Thenthermal convection can occur from the heat source of the shoulder 26 inthe space above the shoulder 26 of the fiber preform 13. This thermalconvection becomes conspicuous when the temperature of the preformcontainer 21 becomes less than 300° C.

In the present embodiment, as described previously, the control unit 29monitors the temperature of the preform container 21 by the temperaturesensors 30. When the measured temperature becomes lower than apredetermined temperature, the control unit 29 actuates the heatermoving device 33 to move the auxiliary heaters 25 before they abut onthe outer wall of the preform container 21 as shown by the chaindouble-dashed lines in FIG. 3. Then the auxiliary heaters 25 areactivated to heat the preform container 21 and thereby heat theatmosphere in the upper chamber 20, thereby preventing theaforementioned occurrence of thermal convection.

The temperature of the preform container 21 at this time needs to beadjusted to not less than 300° C. and it is preferable to adjust thetemperature to not less than 400° C.

By adjusting the quantity of heat dissipation from the outer wall of thepreform container 21 in this way, the present embodiment can finelyadjust the temperature of the preform container 21 and, in turn, thetemperature of the atmosphere in the upper chamber 20 inside thereof andthus prevent the occurrence of thermal convection and the overheat ofthe preform container 21 and the shoulder 26 of the fiber preform 13. Asa consequence, the drawing of the optical fiber 14 can be carried out ona stable basis.

FIG. 5 is a cross-sectional view of the upper preform container part inthe second embodiment of the fiber drawing furnace according to thepresent invention. In the present embodiment the auxiliary heater 25 ais constructed by attaching a heating wire coated with an electricinsulator and a heat resistant alloy, to the outer wall of the preformcontainer 21. Only heat insulators 28 a are arranged movable by aninsulator moving device 33 a. This insulator moving device 33 a is adevice similar to the heater moving device 33 in the first embodiment(see FIG. 3).

In the present embodiment, the clearance 32 between the heat insulators28 a and the auxiliary heater 25 a is adjusted by moving the heatinsulators 28 a between the heating position shown by chaindouble-dashed lines and the dissipating position shown by solid lines,by the insulator moving device 33 a. This implements adjustment of heatdissipation from the external wall of the auxiliary heater 25 a, so asto be able to adjust the quantity of heat dissipation from the preformcontainer 21, whereby the drawing of the optical fiber 14 can beperformed on a stable basis.

In these embodiments, each of the auxiliary heaters 25 or the heatinsulators 28 a has the cross-sectional shape of semicircular archesformed by dividing a cylinder into two parts. However, the shape of theauxiliary heaters 25 or the heat insulators 28 a is not limited to this,but they may be formed in the shape obtained by dividing a cylinder intothree or more parts in the circumferential direction. Another applicableconfiguration is a C-shaped sectional structure obtained by cutting outpart of a side face of a cylinder along the longitudinal directionthereof, and the clearance 32 to the preform container 21 is expanded byelastic deformation of the cylinder while widening the width of this cutpart.

FIG. 6 is a cross-sectional view of the upper preform container 21 inthe fiber drawing furnace of the third embodiment having the auxiliaryheater 25 of this type. Namely, the auxiliary heater 25 in the presentembodiment has the approximately cylindrical shape of the C-shapedsectional structure. Then the width of the clearance 31 of the cut partis expanded by driving the heater moving device 33. This causes theelastic deformation of the auxiliary heater 25 and moves the heater fromthe heating position shown by chain double-dashed lines to thedissipating position shown by solid lines, thereby expanding theclearance 32 between the preform container 21 and the auxiliary heater.The quantity of heat dissipation from the preform container 21 isadjusted in the same manner as in the first and second embodiments byadjusting this clearance 32, whereby the drawing of the optical fiber 14can be performed on a stable basis.

FIG. 7 is a diagram to show a specific example of the heater movingdevice 33 in this embodiment. A belt 333 is wrapped around the auxiliaryheater 25 and the clearance 32 between the preform container 21 and theauxiliary heater 25 is adjusted by controlling the distance of the cutpart of this belt 333 by motors 330 a and shafts 331 a.

FIG. 8 is a cross-sectional view of the upper preform container part inthe fourth embodiment of the fiber drawing furnace according to thepresent invention. In the present embodiment the auxiliary heater 25 ais constructed by attaching a heating wire coated with an electricinsulator and a heat resistant alloy, to the external wall of thepreform container 21 in the same manner as in the second embodiment.Only a heat insulator 28 a having the C-shaped cross section similar tothat in the third embodiment is arranged movable by the insulator movingdevice 33 a. This insulator moving device 33 a is a device similar tothat in the second embodiment.

In the present embodiment the clearance 32 between the insulator 28 aand the auxiliary heater 25 a is also adjusted by expanding theclearance 31 in the cut part of the insulator 28 a by the insulatormoving device 33 a. This permits the quantity of heat dissipation fromthe preform container 21 to be adjusted by controlling the dissipationfrom the external wall of the auxiliary heater 25 a, whereby the drawingof the optical fiber 14 can be performed on a stable basis.

These first to fourth embodiments all are arranged to adjust the heatdissipation from the external wall of the preform container 21 bynatural air cooling, but it is also possible to make use of forcedcooling. FIG. 9 and FIG. 10 respectively present a cutaway structuralview and a cross-sectional view along a line X—X of the major part ofthe fifth embodiment of the fiber drawing furnace of the presentinvention making use of the forced cooling.

In the present embodiment the auxiliary heater 25 is of a cylindricalform, different from the auxiliary heaters 25 in the first and thirdembodiments. The clearance 32 of about 5 cm is formed between the heaterand the preform container 21, also different from the auxiliary heaters25 a in the second and fourth embodiments. Annular heat insulators 34are placed in close fit with the preform container 21 at the upper andlower ends, respectively, of the auxiliary heater 25, so as to seal theclearance 32. Since the present embodiment employs the structure inwhich the heating wire 27 of the auxiliary heater 25 is exposed to theclearance 32, it is preferable to use a nickel-chromium-based oriron-chromium-based heating body with excellent oxidation resistance.

The auxiliary heater 25 is equipped with an inlet tube 35 penetratingthe heater from the outside thereof into the internal clearance 32 andan exhaust tube 36 penetrating the heater from the internal clearance 32to the outside thereof. Shutters 38, which can be controlled to beopened or closed simultaneously, are disposed at respective opening endsof the inlet tube 35 and the exhaust tube 36 outside the cylinder andtheir open/close positions are controlled by a shutter driving device 37actuated by the control unit 29. An air-providing pump 50 is furtherconnected through a supply tube 51 to the inlet tube 35 so as to be ableto blow air into the clearance 32.

During the fiber drawing operation of the optical fiber by use of thepresent embodiment, the control unit 29 monitors the temperature of thepreform container 21 by the temperature sensors 30 in the same manner asin the other embodiments. When the temperature of the preform container21 is about to become not less then 700° C., the power supply isterminated to the auxiliary heater 25 (heating wire 27), the shutters 38are opened, and the air-providing pump, 50 is actuated to supply thecooling air of as large as 5 m³ per minutes through the supply tube 51and inlet tube 35 into the clearance 32. The blowing air forcedly coolsthe preform container 21 from the outside, so that the temperature ofthe preform container 21 is maintained not more than the predeterminedtemperature. As a result, the hot air is discharged through the exhausttube 36 into the atmosphere.

On the other hand, when the temperature of the preform container 21 isabout to become not more than 400° C., the air-providing pump 50 isstopped and the shutters 38 are closed, so as to keep the clearance 32perfectly in a hermetic state. Then the power is supplied to theauxiliary heater 25 (heating wire 27), to heat the preform container 21from the external wall.

In the present embodiment, as shown in FIG. 10, the cooling air issupplied from the tangential direction into the clearance 32 to make aturning flow in the clearance 32, thereby enhancing the heat dissipationeffect from the outside wall of the preform container 21. Besides it, itis also possible to accomplish the same effect by partitioning theinside of the clearance 32 in spiral structure.

The cooling fluid for the forced cooling can also be either of liquidssuch as water, oil, and the like, as well as the air. FIG. 11 is a viewto show the schematic structure of the major part of the sixthembodiment of the fiber drawing furnace of the present inventionemploying this liquid cooling method.

In the present embodiment a heat-transfer plate 39 made of stainlesssteel or the like is attached to the periphery of the auxiliary heater25. A tube is further wrapped around this heat-transfer plate 39 to forma cooling coil 40 and a heat insulator 41 is placed so as to surroundit.

One end of the cooling coil 40 is connected through a flow control valve42 and a pump 43 to a water tank 44, and the other end is connectedthrough a condenser 45 to this water tank 44, thus forming a circulationpath. Cooling water W is reserved in this water tank 44 and an air vent46 is provided at the upper end of the water tank 44. An air pump 48 isconnected through a switching valve 47 to between the cooling coil 40and the flow control valve 42. The control unit 29 controlsopening/closing of these flow control valve 42 and switching valve 47and the operation of the pumps 43, 48.

During the fiber drawing operation of the optical fiber in the presentembodiment, the control unit 29 monitors the temperature measured by thetemperature sensors 30. When the temperature of the preform container 21increases to raise the necessity of heat dissipation, the control unit29 closes the switching valve 47, activates the pump 43, and controlsthe flow regulator valve 42 to let the cooling water W flow at apredetermined flow rate in the cooling coil 40, thereby forcedly coolingthe preform container 21 and promoting heat dissipation from the upperchamber 20.

The cooling water W heated to evaporate during the cooling of thepreform container 21 is condensed by the condenser 45 and then returnedto the water tank 44. Even if the returned cooling water W at the hightemperature increases the temperature of the water inside the water tank44, vapor will be discharged through the air vent 46 whereby thepressure inside the water tank 44 is maintained at the atmosphericpressure.

When the temperature of the preform container 21 becomes too low on theother hand, the control unit 29 stops the operation of the pump 42,closes the flow control valve 42, opens the switching valve 47, andactivates the air pump 47 to force air into the cooling coil 40, therebydischarging the cooling water W remaining in the cooling coil 40 towardthe condenser 45. After that, the control unit stops the air pump 47 andcloses the switching valve 47 to keep the inside of the cooling coil 40in a hermetic state. Then the control unit energizes the heating wire 27to heat the preform container 21 from the outside, so as to maintain thetemperature not less than the predetermined temperature.

This embodiment described the example in which the cooling coil 40 wasdisposed outside the auxiliary 251 heater 25, but the cooling coil 40may be disposed inside the heat insulator 28 of the auxiliary heater 25.

The inventors actually conducted the fiber drawing from the long fiberpreform 13 (having the length of 1.8 m and the diameter of 9 cm) withadjusting the temperature of the preform container 21 in the range of400 to 700° C., using these fiber drawing furnaces. It was verified fromthe result of the fiber drawing that the optical fiber 14 was able to beproduced with less diameter fluctuations, i.e., in the diameter of 125μm ±0.1 μm throughout the entire length (900 km).

Industrial Applicability

The fiber drawing furnaces and fiber drawing method according to thepresent invention are suitably applicable to stable fabrication of theoptical fiber with less diameter fluctuations, particularly, by use ofthe long fiber preform.

What is claimed is:
 1. An optical fiber drawing method comprising stepsof setting an optical fiber preform in a furnace core tube and a preformcontainer connected to an upper portion of the furnace core tube anddrawing an optical fiber from one end of said preform by heating andsoftening, wherein: an upper portion of said preform container isprovided with an auxiliary heater and cooling means for cooling saidupper portion of said preform container and said drawing step includesadjusting a cooling quantity by said cooling means; and said auxiliaryheater is disposed movable relative to an outer wall of said preformcontainer and said adjustment of the cooling quantity by said coolingmeans includes adjustment of a distance between the outer wall of saidpreform container and said auxiliary heater.
 2. An optical fiber drawingmethod comprising steps of setting an optical fiber preform in a furnacecore tube and a preform container connected to an upper portion of thefurnace core tube and drawing an optical fiber from one end of saidpreform by heating and softening, wherein: an upper portion of saidpreform container is provided with an auxiliary heater and cooling meansfor cooling said upper portion of said preform container, and saiddrawing step includes adjusting a cooling quantity by said coolingmeans; and said auxiliary heater is placed on an outer wall of saidpreform container, a heat insulator is disposed movable around theauxiliary heater, and said adjustment of the cooling quantity by saidcooling means includes adjustment of a distance between the outer wallof said preform container and said heat insulator.
 3. An optical fiberdrawing furnace comprising a furnace core tube through which a fiberpreform penetrates vertically, a heater disposed around said furnacecore tube, and a preform container connected to an upper portion of saidfurnace core tube so as to be integral with said furnace core tube toform a semi-closed space opening in part at a lower end, for housingsaid fiber preform inside, said fiber drawing furnace furthercomprising: an auxiliary heater disposed at an upper portion of saidpreform container; cooling means for cooling the upper portion of saidpreform container; and at least one temperature sensor for measuring aninternal temperature in the upper portion of said preform container,wherein said cooling means includes a control unit for adjusting thecooling quantity, based on the temperature measured by said temperaturesensor, wherein said cooling means is air-providing means for supplyingcooling air into clearance between said auxiliary heater and an outerwall of said preform container.
 4. An optical fiber drawing furnacecomprising a furnace core tube through which a fiber preform penetratesvertically, a heater disposed around said furnace core tube, and apreform container connected to an upper portion of said furnace coretube so as to be integral with said furnace core tube to form asemi-closed space opening in part at a lower end, for housing said fiberpreform inside, said fiber drawing furnace further comprising: anauxiliary heater disposed at an upper portion of said preform container;cooling means for cooling the upper portion of said preform container;and at least one temperature sensor for measuring an internaltemperature in the upper portion of said preform container, wherein saidcooling means includes a control unit for adjusting the coolingquantity, based on the temperature measured by said temperature sensor,wherein said cooling means comprises heater moving means for moving saidauxiliary heater to change a distance relative to said preformcontainer.
 5. An optical fiber drawing furnace according to claim 4,further comprising air-providing means for supplying cooling air intoclearance between said preform container and said auxiliary heater,created by movement of said auxiliary heater.
 6. An optical fiberdrawing furnace comprising a furnace core tube through which a fiberpreform penetrates vertically, a heater disposed around said furnacecore tube and a preform container connected to an upper portion of saidfurnace core tube so as to be integral with said furnace core tube toform a semi-closed space opening in part at a lower end, for housingsaid fiber preform inside, said fiber drawing furnace furthercomprising: an auxiliary heater disposed at an upper portion of saidpreform container; cooling means for cooling the upper portion of saidpreform container; and at least one temperature sensor for measuring aninternal temperature in the upper portion of said preform container,wherein said cooling means includes a control unit for adjusting thecooling quantity, based on the temperature measured by said temperaturesensor, wherein said auxiliary heater comprises a heating element and aheat insulator formed around the heating element, said cooling means isinsulator moving means for moving said heat insulator to change adistance relative to said preform container.
 7. An optical fiber drawingfurnace according to claim 6, further comprising air-providing means forsupplying cooling air into clearance between said heat insulator andsaid auxiliary heater, created by movement of said heat insulator.